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Antigenic (genetic) drift--antigenic shift gives rise to diversity after the reassortment of whole genome segments. This can happen when a single cell is infected by two very similar, but distinct, segmented RNA viruses. Antigenic shift can give rise to devastating, worldwide outbreaks of disease because the human population is completely naive to the emerging virus.

Antigenic (genetic) shift--antigenic drift gives rise to diversity through single, random point mutations from errors in viral replication. These mutations are advantageous to the virus if the immune system can not recognize them. They are generally responsible for seasonal outbreaks of influenza.

Epidemic--caused by a disease that is introduced into a community from the outside and affects many people.

Hemagglutinin--
is the surface glycoprotein on the surface of influenza virus A and B. It uses this glycoprotein to infect host cells through the binding of cell’s sialic acid-containing receptors. It is also able to fuse host and viral endosomal membranes.

Neuraminidase--like hemmaglutin it is a glycoprotein that resides on the surface of the viral envelope. By cleaving the terminal sialic acid residues from carbohydrates on the surface of infected cells it is able to promote the release of virus progeny from infected cells. It is also able to prevent aggregation of viruses, This is the receptor that is targeted by neuraminidase inhibiting antiviral drugs.

Reye’s syndrome--a rare but life-threatening complication of encephalopathy and liver degeneration. It primarily affects children following viral infection, usually with influenza B or chicken pox. The administration of aspirin to reduce fever during these viral infections is associated with the development of Reye’s syndrome.